In various industries it is necessary to produce a metal body having a surface with uniform roughness characteristics. In construction, architecture and special purpose applications, for example, it is desired that metal bodies used as window and door frames, railings, curtain walls, and light standards, among other things, include decorative and protective coatings having a smooth, flat, low-gloss, enamel-like finishes that do not have a metallic sheen. Such a metallic sheen results from the light reflective characteristics of the surface which are in turn dependent upon the surface's degree of roughness. It is also desirable to produce protective coatings having such finishes that are not only clear and colorless but also in a variety of colors, including white, gray, bronze, black, red and gold. In lithography, it is desirable that metal body surfaces uniformly have a desired degree of roughness to enable ink or other decorative coatings or finishes to adhere to desired portions of the metal body.
One approach to produce metal bodies with protective and decorative coatings for construction, architecture and special purpose applications is to paint the metal body, which hides the metallic appearance of the coating. Painted aluminum, while performing satisfactorily for many years, does not have the service life, especially in outdoor exposure, that anodized aluminum does. Because the appearance of painted aluminum is popular with architects, and because there is a need to have coatings with a longer service life than paints, it is desirable to develop a protective coating that has the appearance of a painted finish but has the long-term protective quality of protective coatings formed by anodizing. As used herein, anodize refers to a process wherein a metal body is electrochemically treated to produce a coating on the metal body's surface.
Another approach to produce protective and decorative coatings is to produce oxidized coatings by anodizing the metal body. The first step in this process is a chemical etch step to roughen the surface of the metal body before the coating is applied. The etch step is commonly done using a solution containing 50 to 60 grams per liter of caustic soda at 55.degree. to 65.degree. C. The metal body is immersed for 10 to 12 minutes in the etch solution during which time aluminum is dissolved from the surface. The etch typically produces a metal body surface with a pit diameter of 5 to 20 microns and a depth less than 1 micron. When the desired amount of aluminum is removed, the metal body is de-smutted, rinsed and anodized to produce the protective and decorative oxide coating.
This approach suffers from a series of problems. First, the etch step provides a metal body surface not uniformly having a desired degree of roughness. The surface often contains defects such as pit defects, plateau defects, and die lines which cause complications in later process steps. As a consequence, the light reflective characteristics of the metal body surface give the surface a metallic sheen. Second, the processes provide metal bodies with surface roughnesses that are difficult to reproduce. Accordingly in manufacturing operations, the final products do not have uniform color characteristics. Third, the processes require high current densities and voltages. The processes also require temperatures of the electrolytic bath that are so low that they typically require chilling of the bath. Accordingly, the coatings are expensive to produce.
There are numerous processes to provide oxide coatings in desired colors. One process is disclosed in U.S. Pat. No. 3,031,387 to Deal et al., U.S. Pat. No. 3,328,274 to Bushey et al., and United Kingdom Patent No. 1,344,192. U.S. Pat. No. 3,031,387 discloses a process in which the metal body is chemically etched in a sodium hydroxide and sodium fluoride solution and subsequently anodized in an electrolyte consisting of sulfosalicylic acid and at least one substance selected from the group consisting of metal sulfates and sulfuric acid. U.S. Pat. No. 3,328,274 discloses a process in which the metal body is anodized under constant current density and then, when a certain pre-selected voltage is reached, under constant voltage. United Kingdom 1,344,192 discloses a process to control the color of the coating by developing a voltage-time relationship for a given bath composition and temperature and for a particular metal body composition. These processes produce coatings in a variety of colors including gray, bronze, and black. The processes not only require high power requirements but also are extremely sensitive to the composition of the metal body.
Another process uses an oxalic acid solution containing dissolved titanium in the electrolysis step to produce a white coating. Although the coatings produced by this process are white and opaque, the anodizing bath is difficult to maintain and the process hard to control. Consequently, the coatings are not only expensive to produce but also do not uniformly have desired color characteristics. This lack of uniformity exists not only for a given metal body but also among a series of metal bodies subjected to the process.
The lithographic industry by contrast employs an electroetch step and an etch step to provide a roughened metal body surface. U.S. Pat. No. 3,963,594, for example, discloses a process in which aluminum bodies are electrolytically treated under alternating current in an aqueous solution of hydrochloric acid and gluconic acid. The aluminum bodies are thereafter etched in a sodium hydroxide solution at room temperature. Similarly, United Kingdom Patent 1,027,695 discloses a process in which the electroetch step is followed by a brief electrolysis step under direct current to apply a coating to the aluminum body typically having a thickness of from 1 to 2 microns.
These processes also suffer from a series of drawbacks. First, the combined electroetch and etch steps provide a metal body surface not having a uniform degree of roughness. The surface often contains defects which cause complications in later processing. Second, the processes provide metal bodies with surface roughnesses that are difficult to reproduce. Accordingly in manufacturing operations, the final products do not have uniform roughness characteristics.
In view of the above, a need exists for a new method for providing metal bodies having surfaces with a uniform degree of roughness, for the reproducing such surface characteristics, and producing protective and decorative coatings of different colors that have smooth, flat, lowgloss, enamel-like finishes.